1. Field of the Invention
The present invention relates to a method of forming a solid lubricating film on the surface of a substrate of a ceramic material and a ceramic member having a solid lubricating film. More particularly, the present invention relates to a ceramic member such as a ceramic bearing which is used for various precision appliances such as a semiconductor manufacturing apparatus, an electronic appliance and a space appliance, and a method of forming the solid lubricating film of such a ceramic member.
2. Description of the Prior Art
In order to provide lubricating properties for a sliding member of a ceramic material, the surface of the ceramic material is conventionally coated with a film of a solid lubricant. In this case, a material having a lamellar structure such as molybdenum disulfide, tungsten diselenide, graphite and lead oxide is used as the solid lubricant. When such a solid lubricant is formed into a film for covering the surface of the sliding member, a method of mixing the particles of the solid lubricant with an organic or inorganic binder, applying the mixture onto the surface of the sliding member, and baking the sliding member, for example, is adopted.
It is desirable that the solid lubricating film of a sliding member used for a precision appliance is as thin as possible and, at the same time, has a long life. This is because in a precision appliance, even a sliding member is requested to maintain a high dimensional accuracy. One of the most important element for forming a solid lubricating film on a ceramic material is the adhesion between the film and the material, because the higher the adhesion is, the longer becomes the life of the film. Therefore, if the life of the film to be obtained is the same, the higher the adhesion is, the smaller is the thickness which the film is required to have.
It is known that it is possible to form a carbon film on a desired substrate by coating the substrate with an organic resin and irradiating the substrate with ions. For example, Bernard A. Matthiver et al. adopt this method for the surface of a metal and provide a method of improving the resistance to damage of the metal, as disclosed in Japanese Pat. Laid-Open No. 159168/1985.
Bernard A. Matthiver et al. also provide a method of multiple irradiation for enhancing the adhesion between a carbon film and the metal of a substrate, in which after ions are irradiated with a first energy, ions are irradiated again with a second energy which is smaller than the first energy. What is important in this method is that the carbon film obtained by this method is useful only for the materials having the surface characteristics which are metallurgically, namely, chemically fitted for the carbon film, as pointed out by Bernard A. Matthiver. In other words, the material of the substrate is restricted to a metal which can produce a compound with carbon, e.g., an Fe-base alloy.
This method requires a large number of steps for coating and a fine adjustment is required in each step in order to obtain a good film. In addition, in order to make a film having a long life, in other words, a film resistible to a number of times of sliding operations, the film is ordinarily required to have a thickness of not less than 10 .mu.m.
If ions are irradiated on the ceramic surface after it has been coated with an organic resin, the adhesion is insufficient even if a multi-irradiation method is adopted. One reason for this is considered to be that the carbon film and the ceramic material do not fit together metallurgically. As a result of studies, the present inventors have found that another reason for the conventional insufficient adhesion is that when the organic resin film which has been formed in advance, for example, by applying the organic resin on the ceramic substrate is carbonized by ion irradiation, the resin film largely changes in the density, in other words, contracts, thereby producing a large strain stress on the interface between the carbon film and the substrate. It is therefore expected that if the generation of the residual stress on the interface is suppressed as much as possible, it will be possible to form a carbon film having a high adhesion by ion irradiation even on a ceramic surface.
As a result of the studies undertaken by the present inventors on the basis of this view, it has been found that if the deposition of an organic material and ion irradiation are simultaneously carried out, it is possible to form a carbon film having a good adhesion even on a ceramic surface.